The present invention relates to seals in turbines and particularly relates to seals for substantially minimizing or eliminating leakage losses between a turbine nozzle retaining ring and shroud segments.
In a gas turbine, hot gases of combustion flow from combustors through first-stage nozzles and buckets and through the nozzles and buckets of follow-on turbine stages. The first-stage nozzles typically include an annular array or assemblage of cast nozzle segments each containing one or more nozzle stator vanes per segment. Each first-stage nozzle segment also includes inner and outer band portions spaced radially from one another. Upon assembly of the nozzle segments, the stator vanes are circumferentially spaced from one another to form an annular array thereof between annular inner and outer bands. A nozzle retaining ring coupled to the outer band of the first-stage nozzles supports the first-stage nozzles in the gas flow path of the turbine. An annular nozzle support ring, preferably split at a horizontal midline, is engaged by the inner band and supports the first-stage nozzles against axial movement.
During operation and/or repair of the first-stage nozzle, it has been found that warpage can leave gaps between the sealing surfaces of the nozzle retaining ring and the shroud segments. These gaps enable leakage between those axially confronting surfaces. Typically, the confronting faces of the nozzle retaining ring and the shroud segments are provided with W-seals to prevent leakage. However, the W-seals can be jammed during assembly and may crack during operation due to low cycle fatigue. Consequently, there is a need for a new seal which will tolerate the warpage of the various parts of the first stage of the turbine resulting in leakage paths between the nozzle retaining ring and shroud segments, eliminate that leakage and improve the robustness of the seal at that location.
In accordance with a preferred embodiment of the present invention, there is provided a seal between the nozzle retaining ring and the shroud segments, preferably for the first stage of the turbine which eliminates or minimizes leakage past the confronting surfaces of the retaining ring and shroud segments. The seal includes a seal body which extends in an arcuate cavity in one of the axially opposed sealing surfaces of the nozzle retaining ring and shroud segments radially outwardly of the hot gas path. The seal body has preferably a first, generally U-shaped portion in cross-section, and a pair of reversely extending, generally U-shaped marginal portions in cross-section along opposite sides of the U-shaped portion. With the seal body disposed in the cavity and at turbine operating conditions, the marginal portions of the seal body lie in sealing engagement against an interior surface of the cavity, e.g., the base of the cavity, and the opposite opposed axially facing sealing surface whereby any gap otherwise affording leakage flow past these confronting axially facing surfaces is substantially eliminated.
In a particular preferred form of the present invention, the seal is formed of sheet metal, preferably a pair of complementary-shaped sheet metal plates secured, for example, by welding, to one another and bent into the aforementioned cross-sectional configuration. To install the seal, the seal is first placed in a compressed state and maintained in that compressed state during installation. To accomplish this, the seal may be wrapped by a material which, at turbine operating or near-operating conditions such as temperature, disintegrates, releasing the seal to expand in the cavity which, under preload, biases the marginal portions of the seal body against the sealing surfaces. The wrap may be formed of a Kevlar(copyright) 29 or may be formed of a high-strength plastic material, such as Lexan(trademark) or Ultem(trademark) clips to hold the seal in a compressed condition during installation. Alternatively, epoxy may be applied to the compressed seal to maintain the seal in the cavity in the compressed condition, the epoxy releasing the seal body at operating or near-operating turbine conditions for sealing engagement with the opposed sealing surfaces.
In a preferred embodiment according to the present invention, there is provided a gas turbine comprising a turbine nozzle retaining ring having a generally axially facing first surface, a shroud segment having a second surface in axial opposition to the first surface, one of the first and second surfaces defining a cavity opening generally axially toward another of the first and second surfaces and a flexible seal in the cavity including a seal body having a first, generally U-shaped portion in cross-section and a pair of reversely extending, generally U-shaped marginal portions in cross-section along opposite sides of the U-shaped portion, the marginal portions in sealing engagement with an interior surface of the cavity in one surface and another of the first and second surfaces, respectively.
In a further preferred embodiment according to the present invention, there is provided a turbine comprising a turbine nozzle retaining ring having a generally axially facing first surface, a plurality of turbine shroud segments having an annular second surface in axial opposition to the first surface, one of the first and second surfaces having a cavity opening generally axially toward another of the first and second surfaces and at a location radially outwardly of the first seal and a flexible seal in the cavity including a seal body having a first, generally U-shaped portion in cross-section and a pair of reversely extending, generally. U-shaped marginal portions in cross-section along opposite sides of the U-shaped portion, the marginal portions in sealing engagement with an interior surface of the cavity of one surface and another of the first arid second surfaces, respectively.